The present invention relates to a method of treating malodorous gases of a pulp mill, for reducing emissions of harmful nitrogen compounds, such as nitrogen monoxide and nitrogen dioxide, from the mill.
In sulphate (kraft) cooking, cellulose material (usually wood chips) is treated in white liquor, which contains sodium hydroxide and sodium sulphide, so that lignin is hydrolyzed. Consequently, several organic sulphur compounds, such as methylmercoptan, dimethylsulphide and dimethyldisulphide are produced. These compounds, and hydrogen sulphide are the compounds which cause the unpleasant odor of many sulphate pulp mill vent gases. Gases containing these sulphur compounds are formed in several stages of the pulp manufacturing process, such as cooking and spent liquor evaporation. Malodorous sulphur compounds are most usually removed by collecting the malodorous gases from different sources and burning them in either a lime kiln, recovery boiler, or a separate incinerator. As a result of combustion, all sulphurous substances are oxidized to sulphur dioxide, sulphur trioxide and, in the presence of alkali, also to sodium sulphate, and they end up in flue gases. Sulphur dioxide is also formed in other stages of the pulp manufacturing process, e.g. in black liquor combustion. Stricter and stricter limitations have been set on sulphur dioxide emissions from pulp mills, and achieving these ever lower limits has required continuous development of processes and equipment.
Nitrogen oxides is another group of compounds which has received more and more attention, and which are desirably removed from kraft pulp mill off gases. Efforts to reduce emissions of nitrogen oxides have been focused mostly on black liquor combustion, the lime kiln, and the power boiler. Nitrogen compounds originate from either thermal oxidation of nitrogen in the combustion air or reduction and subsequent oxidation of the nitrogen bound to fuel, e.g., black liquor. In the reducing conditions typically prevailing in the furnace of the recovery boiler, in the combustion processes the nitrogen which has been introduced with feed liquor into the furnace is first converted to ammonia and nitrogen compounds ending up in the chemical melt. In conventional combustion or in combustion effected with staged oxidation, i.e., air is introduced from several different levels, such as primary, secondary, tertiary, and quartenary airs ("low NO.sub.X combustion"), this ammonia forms molecular nitrogen and nitrogen oxides that are likely harmful to the environment. Typically, half of the above-identified ammonia is converted to nitrogen oxides and the other half to nitrogen gas. By low NOx combustion (with staged air feed when the understoichiometric conditions turn to overstoichiometeric conditions), conversion of the above-identified ammonia to nitrogen oxides can be reduced, typically by about 20%. Staged air feed and various auxiliary processes attached thereto have enabled considerable lowering of nitrogen oxide emissions from the recovery boiler. Although black liquor combustion is satisfactorily controllable, pulp mills have not been able to lower the total emissions of nitrogen oxides in a desired manner.
According to the present invention, there is provided a method of controlling emissions of harmful nitrogen compounds, especially nitrogen oxides, from a pulp mill in a more efficient way than in the prior art. The invention relates to a method in which malodorous gases of a pulp mill are burned in order to oxidize reduced sulphur compounds, and it is a characteristic feature of the present invention that prior to burning the malodorous gases are treated in order to lower their ammonia content, and ultimately reduce emissions of harmful nitrogen compounds from the mill.
According to one aspect of the invention there is provided a method of reducing emissions of harmful nitrogen compounds from a pulp mill (particularly a kraft mill) having malodorous gases which include ammonia and reduced sulphur compounds therein. The method comprises: (a) Treating the pulp mill malodorous gases to lower the ammonia content thereof; and then (b) burning the malodorous gases under conditions which effect oxidation of the reduced sulphur compounds therein, so as to reduce odor emissions and emissions of harmful nitrogen compounds from the pulp mill.
In the method (b) may be practiced using a dedicated incinerator, a recovery boiler, power boiler, or lime kiln. Preferably (a) is practiced by washing the malodorous gases with a bisulphite solution. Also, preferably (b) is practiced to produce flue gases; and the method further comprises (c) washing the flue gases with a sodium sulphite solution to produce sodium bisulfite; and (a) is practiced by washing the malodorous gases with the bisulphite solution from (c).
Typically the pulp mill includes a steam stripper in which foul condensates of the mill are cleaned to produce clean condensate and discharge malodorous gas; and preferably (a) and (b) are practiced using malodorous gas from the steam stripper as part of the malodorous gases treated. The method also preferably comprises regulating the operating conditions, including the pH, of the stream stripper in order to reduce the ammonia content of the clean condensate.
In the method (a) may be alternatively practiced by washing the malodorous gases with at least one of acid bleaching effluent, and waste acid from a chlorine dioxide plant. Preferably (b) is practiced with at least about a 3% (e g. about 34%) excess of air and at a temperature of over about 800 degrees C. Also (a) and (b) may be practiced using HCLV gases as the malodorous gases.
The pulp mill typically has a waste water purification plant; and the method further comprises, after the practice of (a), transporting the bisulfite solution used for washing to the waste water purification plant. The pulp mill also has a bleach plant; and the method may alternatively or further comprise, after the practice of (a), transporting the bisulfite solution used for washing to the bleach plant.
According to another aspect of the invention, a method of treating kraft pulp mill malodorous gases which include ammonia and reduced sulphur compounds therein is provided, comprising: (a) washing the kraft pulp mill malodorous gases with a bisulfite solution to lower the ammonia content thereof; and then (b) burning the malodorous gases under conditions which effect oxidation of the reduced sulphur compounds therein. In the method (b) is practiced to produce flue gases; and the method further comprises (c) washing the flue gases with a sodium sulphite solution to produce sodium bisulfite; and (a) is practiced by washing the malodorous gases with the bisulphite solution from (c). Other details of this aspect of the invention are also as described above.
It is the primary object of the present invention to enhance the environmental friendliness of pulp mills, particularly kraft pulp mills. This and other objects of the invention will become apparent from the detailed description and appended claims.